This disclosure relates to a method for preselecting a gear in a multi-clutch transmission of a vehicle upon exiting free-wheeling state; the multi-clutch transmission comprising friction clutches drivingly connected to a prime mover, an transmission output shaft and a main transmission that comprises input shafts connected to said friction clutches, a countershaft (parallel to at least one of said input shafts, gearwheels and tooth clutches; wherein the multi-clutch transmission being operated in a free-wheeling state having a gear of said first set of gears preselected; the first friction clutch being disengaged. The method according to the disclosure may be applied to all multi-clutch transmissions, and is consider particularly suitably for use in heavy vehicles such as trucks, buses, construction vehicles and the like.
In principle, a dual clutch transmission has two input shafts, each connectable to a friction clutch and to the output of the engine. Functionally, this is equivalent to having two conventional transmissions in parallel, i.e., two parallel sub-transmissions, and using one at a time for power transfer.
The sub-transmission that is not used, idling, for the time being can have a gear engaged and prepared, i.e. preselected, for a subsequent gear shift. The subsequent gear shift is carried out by simultaneously disengaging the friction clutch of the previously used sub-transmission and engaging the friction clutch of the previously idling sub-transmission.
Normally, in a dual clutch transmission, gears are preselected in the presently idling sub-transmission by engaging and disengaging tooth clutches. For a smooth and durable operation, this requires that the parts to be engaged by a tooth clutch are synchronized, i.e., that they have fairly equal rotational speed. If not, the clutch teeth would clash, resulting in worn or broken teeth and noise. A mismatch in rotational speed will also result in torque disturbance felt by the driver. So, different kinds of devices and arrangements are used for synchronizing the parts to be engaged. A straightforward solution is to have every tooth clutch in the transmission designed as synchronizers, i.e., be equipped with synchronizing clutch elements, e.g. as in US2008/0188342A1. However this solution has the disadvantage of relatively high manufacturing costs.
An alternative solution for synchronising of the transmission is use of a central synchronizing unit. Basically, only two synchronizing devices are required one that can make the speed of the first sub-transmission larger than that of the second sub-transmission, and one that can make the speed of the first sub-transmission less than that of the second. That will work when the first sub-transmission is idling and the second is active, as well as when the first sub-transmission is active and the second is idling. Such a device is referred to as a central synchronizing unit. The central synchronizing unit eliminates the need for providing each individual tooth clutch with a separate synchronisation surface and may consequently result in reduced overall manufacturing cost of the transmission. However, all synchronisation relying on friction surfaces for equalising the rotational speed of different parts of the transmission inherently results in energy losses when kinetical energy of the rotating parts is converted into heat.
There is thus a need for an improved method for preselecting gears in multi-clutch transmissions removing the above mentioned disadvantage.
It is desirable to provide a method for preselecting a gear in a multi-clutch transmission upon exiting free-wheeling state, where the previously mentioned problem of energy loss caused by synchronization is at least partly avoided.
The disclosure concerns a method for preselecting a gear in a multi-clutch transmission of a vehicle upon exiting free-wheeling state. The multi-clutch transmission comprises friction clutches drivingly connected to a prime mover, a transmission output shaft, and a main transmission that comprises input shafts connected to the friction clutches, a countershaft parallel to at least one of the input shafts, gearwheels and tooth clutches, where a first clutch of the friction clutches and a first subset of the tooth clutches are arranged to be selectively engaged in order to transfer torque in a first set of gears between the prime mover and the transmission output shaft, and where a second clutch of the friction clutches and a second subset of the tooth clutches are arranged to be selectively engaged in order to transfer power in a second set of gears between the prime mover and the transmission output shaft. The multi-clutch transmission when being operated in a free-wheeling state has a gear of the first set of gears preselected and the first friction clutch disengaged.
The inventive method comprises the steps of:
A) upon determining that at least one predetermined condition for exiting free-wheeling state is met; increasing the rotational speed of the prime mover towards a rotational synchronization speed of said preselected gear; the second friction clutch being in a closed state such that the rotational speed of the input shaft of the second friction clutch increases with the increasing rotational speed of the prime mover;
B) disengaging the second friction clutch;
C) engaging, at a suitable time point, at least one tooth clutch of said second subset of said tooth clutches corresponding to a gear of said second set of gears such that said gear of said second set of gears becomes preselected.
By carrying out the steps of the inventive method synchronizing elements such as a central synchronizing unit does not need to be activated to the same extent as the prior art solution in order for the countershaft to achieve the same rotational speed as the main shaft prior to engaging a tooth clutch of the second set of tooth clutches. An increase in the rotational speed of the countershaft is instead achieved preferably primarily by having the counter shaft rotationally connected to the prime mover during the engine speed increase following a freewheeling state, and the subsequent engagement of the tooth clutch of second subset of the tooth clutches being actuated once the rotational speed of the counter shaft has increased a certain extent. The inventive method thus reduces the degree of frictional synchronisation needed to preselect the gear of the second set of gears, thereby immediately reducing the level of energy dissipated as heat by the synchronisation device. The invention according to the inventive method consequently reduces the fuel consumption of the vehicle, reduces service costs of the clutch, and potentially increases the reliability compared with the prior art solution.
According to an aspect of the invention, the step of disengaging the second friction clutch is performed when the rotational speed of the input shaft of the second friction clutch has reached at least a predetermined percentage of the rotational synchronization speed of the gear of said second set of gears. Disengagement of the second friction clutch must be triggered by an event, and upon reaching at least a predetermined percentage of the rotational synchronization speed of the gear of said second set of gears is a good trigger point because this strategy is equally applicable for preselected of different gears, i.e. is applicable upon exiting freewheeling state with different vehicle speeds. The percentage is preferably selected to enable direct connected of the relevant tooth clutch or tooth clutches without further frictional synchronisation needed. Alternatively, step of disengaging the second friction clutch is performed when the rotational speed of the input shaft of the second friction clutch has reached at least a predetermined value. Alternatively, the step of disengaging the second friction clutch is performed when at least a predetermined time period has passed after begin of increasing the rotational speed of the prime mover.
According to an aspect of the invention, disengagement of the second friction clutch is preferably realised when the rotational speed of the input shaft of the second friction clutch has reached a rotational speed which is higher than the rotational synchronization speed of the gear of said second set of gears. Thereby, the frictional synchronisation is not necessary and the control system may select an appropriate time point for engaging the tooth clutch of the gear of the second set of gears, such that a jerk-free preselection is performed.
According to an aspect of the invention, disengagement of the second friction clutch is performed when the rotational speed of the input shaft of the second friction clutch has reached a rotational speed which is lower than the rotational synchronization speed of said gear of said first set of gears. There is no need to increase the rotational speed of the input shaft of the second friction clutch more than that.
According to an aspect of the invention, after disengagement of the second friction clutch and prior to engagement of at least one tooth clutch of said second subset of said tooth clutches, synchronizing by means of mechanical synchronizing means is performed, such that the speed of the input shaft of the second friction clutch becomes identical or nearly identical with the rotational synchronization speed of a gear of said second set of gears. Frictional synchronisation may be required in case the speed difference between the rotational speed 42 of the second input shaft and the rotational synchronisation speed 33 of the gear of the second set of gears is too large.
The drop in rotational synchronization speed of the input shaft of the second friction clutch towards that of a gear of the second set of gears is affected by the friction forces acting upon the countershaft. Thereby, the drop in rotational speed is slow and may be monitored and controlled such that the tooth clutch is brought to an engaged state when the speed difference is relatively small, for example in the range of 40-120 rpm. Engagement of the tooth clutch at the differential speed of the gear of the second set of gears reduces the risk of tooth-tooth wear and tear and contributes to rapid gearshifts.
A predetermined condition for determining that the multi-clutch transmission is ready to exit free-wheeling state may be that the rotational speed of the output shaft, connected to the wheels, is decreasing below a set value i.e. the hill is flattening out such that the speed/momentum of the vehicle drops.
The time point for starting increasing the rotational speed of the prime mover towards the rotational synchronization speed of the preselected gear is preferably chosen such as to achieve a balance between fuel savings made by a vehicle travelling in free-wheeling state and loss in momentum of the same vehicle e.g. resulting from travelling on a road flattening out after going downhill.
In free-wheeling state, and upon exiting free-wheeling, the second friction clutch may be in an engaged state. The second friction clutch may for example by connected to the drive shaft of an engine oil pump, or the like, such that engine power during idling in the free-wheeling state is required to be more or less continuously transferred via the second friction clutch. The degree of engagement can vary. For example, the second friction clutch may be in a fully engaged state, i.e. in its maximal or near maximal torque transfer capacity state. Alternatively, the second friction clutch may be in a semi-engaged state, i.e. in a state where torque transfer capacity is reduced. This engagement state can be advantageous at engine idling conditions and the like to prevent accidental stall of the engine in case of non-optimal engagement timing of the second friction clutch. Still more alternatively, the second friction clutch may be in a disengaged state.
Generally when the second friction clutch is in an engaged state during freewheeling state torque transfer between the prime mover and the transmission output shaft is disabled. In such a state, the input shaft of the second friction clutch is not connected to the transmission output shaft, i.e. no gear is selected to enable power transfer via the second friction clutch in the freewheeling state.
The method may also comprise a method step D); engaging the first friction clutch when the rotational speed of the prime mover reaches the rotational synchronization speed of the preselected gear of the first set of gears such that torque transfer from prime mover to the transmission output shaft is enabled. Hence, the preselected gear of the first set of gears is enabled. This step is preferably carried out after the increase of prime mover speed in step A). It may be performed before, after or simultaneously as step B).
The time point for actuating step D); engaging the first friction clutch to enable torque transfer via the preselected gear of the first set of gears, is determined by the fulfillment of preset conditions such as when the engine speed has stabilised at the rotational synchronization speed of the preselected gear of the first set of gears.
The steps of the method may be carried out in a sequence of A, B, D, C; A, D, B, C or A, B, C, D.
The step B) may be performed when the input shaft of the second friction clutch has reached a rotational speed which is higher than or equal to the rotational synchronization speed of the gear of the second set of gears and lower than the rotational synchronization speed of the gear of the first set of gears. Thereby the rotational speed the input shaft of the second friction clutch may start to slowly drop towards the rotational synchronization speed of the gear of the second set of gears. The rotational speed of the input shaft of the second friction clutch is preferably not dropping to a speed substantially below the rotational synchronization speed of the gear of the second set of gears before performing step C) of engaging the tooth clutch of the second subset of the tooth clutches corresponding to the gear of the second set of gears. Thereby no synchronizer is needed to match the rotational speeds.
If the rotational speed of input shaft of the second friction clutch is increased in step B to a speed which is near the rotational synchronization speed of the gear of the second set of gears, within an engaging range, step C) must be performed without delay in order to avoid dropping below the synchronization speed of the gear of the second set of gears.
The tooth clutch of the second subset of the tooth clutches corresponding to the gear of the second set of gears may be engaged in step C) when the rotational synchronization speed of the input shaft of the second friction clutch is within an engaging range of +/−20% of the synchronization speed of the gear of the second set of gears, preferably within an engaging range of +/−10%, and more preferably within an engaging range of +/−5%. A rotational synchronization speed of the input shaft of the second friction clutch falling within the engaging range of the synchronization speed of the gear of the second set of gears eliminates the risk of tooth-tooth clashes, resulting in worn, or broken, teeth and noise upon engaging the tooth clutch of the second subset of the tooth clutches. The preselection of the gear of the second set of gears is thus performed in a favourable manner, without using a synchronizer and without causing wear and tear of the affected components.
The first friction clutch is a normally open clutch. The second friction clutch is a normally closed clutch.
The multi-clutch transmission is arranged to be capable of shifting from one of the first or second set of gears to one preselected of the other of the first or second set of gears without interruption of the power transfer between the prime mover and the transmission output shaft.
The main transmission is arranged to, while transferring torque in one of the first or second set of gears, preselecting the other of the first or second set of gears.
The multi-clutch transmission used to carry out the steps of the inventive method may be a dual-clutch transmission.
The multi-clutch transmission may comprise a main transmission having six forwards gears, and a range transmission having two gears coupled to the main transmission, such that the multi-clutch transmission comprises a total of twelve forward gears.
Free-wheeling state is defined as the state when the vehicle is allowed to roll without propulsion torque being applied to the wheels from a power source of the vehicle. This is usually accomplished by disconnecting the engine from the driven wheels of the vehicle. Free-wheeling state is for example often enabled by automatic multi-clutch transmissions in heavy vehicles upon travelling downhill.